The present invention relates to a variable displacement pump and more specifically to a variable displacement pump which can be used for an internal combustion engine, to supply an oil to various sliding contact portions, for example.
Recently, oil discharged from an oil pump is used, in addition to lubrication for sliding contact portions of an engine, for various other purposes requiring different discharge pressure levels; for example, a driving source for variable valve actuation apparatus, an oil jet for cooling pistons and for lubrication of bearings supporting a crankshaft. Accordingly, there are demands for oil pumps providing desirable changeover between a low pressure characteristic and a high pressure characteristic in a low rotational speed region, and a high pressure characteristic in a high rotational speed region. To meet these demands, a variable displacement pump is proposed in a patent document 1, JP2011-111926 (≈US 2011/0123379A1)
In the variable displacement pump disclosed in this patent document, a cam ring is urged by two spring members provided with different spring loads, to achieve a lower pressure characteristic and a high pressure characteristic mechanically without using an electric control device.